This invention relates to high definition televison, and in particular to the reception of signals transmitted over different types of transmission channels. The invention is particularly concerned with the reception of frequency multiplexed signals such as PAL or HD-PAL by a receiver designed for reception of time multiplexed signals such as MAC or HD-MAC.
It has been recognised that the introduction of a compatible television service provided via direct broadcasting satellites or other methods of delivering uses a PAL composite method of conveying the colour signal which is not directly compatible with MAC and its derivatives.
In the conventional PAL signal, luminance and colour signals are frequency multiplexed for transmission. By contrast, MAC signals are time multiplexed for transmission and, when separated, give a greater horizontal signal bandwidth than is available with PAL terrestrially broadcast signals. MAC gives an 8 MHz luminance bandwidth whereas PAL may only give 3.5 MHz useable luminance bandwidth at the output of a PAL decoder. By applying complex decoders to the PAL signal, up to 5.5 MHz may be derived.
If an HD-MAC signal were to be transmitted via PAL the additional frequencies derived in the HD-MAC decoder would not be useable because there would be a large gap in the middle of the spectrum. The reason for this is that for a still picture, horizontal frequencies up to 16 MHz are coded into an effective 8 MHz bandwidth by folding them about the 8 MHz frequency. The signal is unfolded in the HD-MAC decoder, and it is vital that the full 8 MHz bandwidth is available at the HD-MAC decoder. However, if the signal is constrained in the communication path such that, for example, 3.5 MHz is available in the luminance channel of the HD-MAC decoder, the unfolded spectral components are only present from 0 to 3.5 MHz and from 12.5 MHz to 16 MHz. In this case, the higher-frequency components are of no value in enhancing the resolution of the picture because of the gap in the middle of the spectrum. Components in the band 3.5 MHz to 7 MHz would, however considerably enhance the resolution of the picture.